There were a few error messages that a user could experience that would, previously, be without any the ability to be localized. There are some more E_USER_ERRORs that I did not change to a constant, for example the error message that is displayed if there aren't any folders in /language.
PHPBB3-9975
Extend the request class with helpers for reading server vars (server())
and HTTP request headers (header()). Refactor the existing code base
to make use of these helpers, make $_SERVER a deactivated super global.
Also introduce an is_ajax() method, which checks the X-Requested-With
header for the value 'XMLHttpRequest', which is sent by JavaScript
libraries, such as jQuery.
PHPBB3-9716
Due to the usage of `__DIR__` for the file includes the tests can't
be ran on systems with PHP < 5.3. Replace all occurances of
`__DIR__` with `dirname(__FILE__)`.
PHPBB3-10011
